Integrated ringer for short telephone lines

ABSTRACT

The invention provides an apparatus for use with a subscriber line interface circuit in a telephone system, the subscriber line interface circuit being interposed between a telephone central office and a subscriber line, the subscriber line being coupled with a subscriber telephone device. The apparatus comprises an interface circuit for coupling the apparatus with the subscriber line; a ringing generator coupled with the interface circuit for providing a time varying signal to the subscriber line in response to a received ring signal; a detector circuit coupled with the interface circuit for detecting an impedance of the subscriber line in the presence of the time varying signal and providing a detect indication when the impedance is below a predetermined impedance threshold; and a control circuit coupled with the ringing generator and with the detector circuit for providing the ring signal to the ringing generator and receiving the detect indication from the detector circuit, the control circuit interrupting the ring signal in response to receiving the detect indication. The apparatus provides balanced ringing signals to the subscriber telephone device and detects loop AC impedance to provide ring trip detection.

BACKGROUND OF THE INVENTION

The present invention generally relates to an apparatus for use with asubscriber line interface circuit in a telephone system. Moreparticularly, the present invention relates to a method and apparatusfor providing a time varying ringing signal to the subscriber line, thesubscriber line being coupled with a subscriber telephone device, thesubscriber telephone device generating a ringing indication in responseto the time varying ringing signal. The invention still further relatesto an apparatus for detecting an off hook state of the subscriber linedevice, and interrupting the time varying ringing signal in response tothe off hook detection.

A telephone system generally includes one or more subscriber telephonedevices, each subscriber telephone device being coupled with a telephonecentral office by a subscriber line. The subscriber telephone devicesmay be telephones or other telephone equipment. The telephone centraloffice handles switching of telephone signals between subscribertelephone devices. Industry standards govern the electrical signallevels used for switching, coupling and signalling within the telephonesystem.

When a telephone call is placed to a particular subscriber telephonedevice, the telephone central office must send signals to the telephonedevice to indicate the incoming call. The telephone central office sendsringing signals which cause the subscriber telephone device to generatea ringing indication. The ringing indication may be a ringing bell, anelectronic tone or some other audible or visible indication of ringing.The telephone central office applies the ringing signals directly to thesubscriber telephone line.

Each subscriber telephone device is coupled to the telephone centraloffice by a subscriber line. The subscriber line generally includes twoconductors, one labelled "tip" and the other labelled "ring." Tipconductors and ring conductors carry both AC (time varying) and DCsignals. The subscriber telephone device plus, the associated tip andring conductors is commonly known as a loop or subscriber loop.

The subscriber telephone device can be modelled electrically as aresistor, an inductor and a capacitor in series and selectively coupledby a switch to the tip and ring conductors. The switch is known as thehook switch. When the telephone is on hook, and not in use, the hookswitch couples the resistor-inductor-capacitor combination between thetip and ring conductors. When the telephone is off hook, or in use, theswitch couples the tip and ring conductors through a resistor only. Bydetecting the D.C. impedance between the tip and ring conductors, thetelephone central office can determine whether the subscriber telephonedevice is off hook (in use) or on hook (not in use).

When providing ringing signals to an intended receiving subscribertelephone device in response to a call originated by another (calloriginating) subscriber telephone device, the telephone central officemust be able to determine whether the intended receiving subscribertelephone device is on hook or off hook for several reasons. First, ifthe intended receiving subscriber telephone device is off hook, thetelephone central office must not send ringing signals, but must rathersend a busy tone to the call originating subscriber telephone device.Secondly, if a user answers the phone by taking the receiving subscribertelephone device off hook in response to the ringing signal, thetelephone central office must detect the change from on hook to off hookso that the receiving telephone does not ring loudly in the user's ear,causing the user discomfort. Further, in response to the receivingtelephone going off hook, the telephone central office completes theconnection between the call originating telephone device and thereceiving subscriber telephone device. The process of providing ringingsignals and detecting the on hook or off hook state of the receivingsubscriber telephone device by the telephone central office is known asring trip detection.

There are numerous industry standards governing ring trip detection. Thetelephone central office, or other equipment providing the ringingsignals to the subscriber lines, must detect that the receivingsubscriber telephone device has gone off hook and terminate the ringingsignals within a predetermined time period, such as 200 msec. Further,the telephone central office or other device which provides ringingsignals to the subscriber line must be able to provide a ringingindication to any telephone coupled to the subscriber line. Thetelephone central office or other device providing the ringing signalsshould provide no false ring trip detections and should not miss anyring trip detections. Still further, the device which provides theringing signals to the subscriber line must work with any length phoneline, including both short (low impedance) or long (high impedance)subscriber lines.

Short subscriber lines couple the telephone central office to subscribertelephone devices which are physically near the central office, perhapsjust a few blocks away. Long subscriber lines couple the telephonecentral office to subscriber telephone devices which are physicallydistant from the central office, perhaps miles away. Long subscriberlines have a greater impedance, measured from the central office, thanshort subscriber lines. Long subscriber lines are also more susceptibleto noise due to coupling from adjacent noise sources such as othersubscriber lines and power lines, than are short subscriber lines.

One industry standard requires the ringing signal to be applied as an ACvoltage with a DC offset. The AC voltage, measured at the telephonecentral office, is preferably a 90 volt rms, 20 Hz, AC signal in orderto provide at least 40 volts vms at the receiving telephone device. TheDC offset is preferably 48 volts, measured at the central office. Theringing signal is generally applied as an unbalanced or single-endedringing signal. That is, the AC signal is applied to either the tipconductor or the ring conductor with either polarity of DC offset(referred to as ring-plus, ring-minus, tip-plus and tip-minus ringing).The conductor to which the AC and DC ringing signals are not applied isgrounded in an unbalanced ringing design. The goal when applying ringingsignals is to place the 90 volt rms AC signal across the tip and ringconductors.

When the subscriber telephone device is on hook, no DC path exists inthe subscriber telephone device to permit DC current to flow in responseto the applied DC offset voltage. However, with the telephone on hook,alternating current may flow in response to the applied AC ringingvoltage to cause the subscriber telephone device to generate a ringingindication. When the subscriber telephone device is taken off hook, a DCpath is established to couple the tip and ring conductors and allow DCcurrent to flow in the loop. The central office detects the flow of theDC current in the loop to determine that the subscriber telephone devicehas been taken off hook and interrupts the ringing signal. Thus, priorart telephone apparatus have detected ring trip by detecting the DCimpedance between the tip and ring conductors, typically by measuring DCcurrent flow in response to a known applied DC voltage.

Application of the 90 volt ms signal and the 48 volt DC offset to thesubscriber line means that the ringing generator must be able to handlepotential differences substantially equal to 250 volts. It hasheretofore been uneconomical to fabricate a ringing generator using asilicon integrated circuit.

Silicon integrated circuits which can handle 250 volts are expensive.Individual circuit elements, such as transistors, resistors andcapacitors, designed to handle such large voltages must be physicallylarge in order to sink and source the large currents associated withsuch large voltages. This large size requires substantial "real estate"on the surface of an integrated circuit which reduces the scale ofintegration of such devices as well as reducing manufacturing yield.Both the large size and the reduced yield increase manufacturing cost.

Moreover, circuit elements which can handle up to 250 volts must befabricated using a manufacturing process designed to produce deviceshaving junction breakdown potentials in excess of this voltage. Deviceswith smaller junction breakdown potentials will not function properlyand may be permanently damaged when subjected to such large voltages.Manufacturing processes for silicon devices of the type commonly usedfor logic and control functions do not provide sufficient junctionbreakdown potentials to handle such large voltages, so logic and controlcircuitry cannot be readily integrated with telephone ringing signaltransmitting circuitry. This further increases manufacturing cost of theoverall system.

Where a central telephone office must generate ringing signals, the 250volt requirement is not prohibitive. The telephone central office may becoupled with many thousands of subscriber lines. A single ringinggenerator is needed to provide ringing signals to these many subscriberlines. Thus, the high cost of the equipment needed to generate the 250volt ringing signals is shared by the many thousands of subscriberlines.

However, modern telephone systems are moving away from systems in whicha central office supplies many thousands of subscriber lines directly.Rather, in modern systems the telephone central office is coupled by anoptical fiber to an optical network unit (ONU). The ONU couples thedigital signals carried by the optical fiber with the analog electricalsignals carried by the subscriber line to the subscriber telephonedevice. A relatively small number of subscriber lines are coupled to theONU, perhaps one to ten. The ONU is located physically close to thesubscriber telephone devices serviced by this small number of subscriberlines, preferably in the same city neighborhood as the subscribertelephone devices. This is known as a fiber in the loop (FITL) system.

In a FITL system, each ONU must be capable of generating ringingsignals. Ringing signals are not carried on the fiber optic line; thefiber optic line only transmits a ring command from the central officeto the ONU. In response to the ring command, the ONU must generate theappropriate ringing signal on the appropriate analog subscriber line.Because only one or a very few subscriber lines are coupled to an ONU,the high cost of a ringing generator capable of supplying 250 voltscannot be shared among a large number of subscriber lines, as was thecase in the prior art central office telephone system. However, sincethe ONU is physically close to the subscriber telephone devices which itservices, the ONU must only provide analog signals to a short subscriberline, which may be only a few city blocks in length.

Accordingly, there is a need in the art for an apparatus and method forgenerating ringing signals requiring a total voltage less than 250 voltsand which will cause a subscriber telephone device to produce a ringingindication. Further, there is a need in the art for an apparatus andmethod for generating ringing signals using only a silicon devicecoupled with the subscriber line. Still further, there is a need in theart for an apparatus and method for detecting ring trip using a silicondevice.

SUMMARY OF THE INVENTION

The invention provides a method for providing ringing signals to asubscriber telephone device coupled to a subscriber line, the subscriberline including a first conductor and a second conductor. The methodincludes the steps of providing a first time varying signal to the firstconductor and a second time varying signal to the second conductor;detecting an AC impedance between the first conductor and the secondconductor in the presence of the first time varying signal and thesecond time varying signal; and providing a ring trip indication whenthe AC impedance is below a predetermined threshold.

The invention further provides an apparatus for use with a subscriberline interface circuit in a telephone system, the subscriber lineinterface circuit being interposed between a telephone central officeand a subscriber line, the subscriber line being coupled with asubscriber telephone device. The apparatus comprises an interface meansfor coupling the apparatus with the subscriber line; a ringer meanscoupled with the interface means for providing a time varying signal tothe subscriber line in response to a received control signal; and detectmeans coupled with the interface means for detecting an impedance of thesubscriber line in the presence of the time varying signal and providinga ring trip indication when the impedance is below a predeterminedimpedance threshold.

The invention further provides an apparatus for coupling a subscriberline with a digital signal path in a telephone system, the apparatusreceiving information including a ring command from the digital signalpath, the subscriber line being coupled with a subscriber telephonedevice. The subscriber device includes signal means and hook switchmeans. The subscriber line has a first conductor and a second conductorand the hook switch means has a first state for coupling the firstconductor with the second conductor and a second state for decouplingthe first conductor and the second conductor. The signal means generatesa ringing indication in response to a ringing signal On the subscriberline when the hook switch means is in the second state. The apparatuscomprises interface means for coupling the apparatus with the subscriberline; ringer means coupled with the interface means for providing theringing signal to the subscriber line, the ringing signal preferablyincluding only a first time varying signal and a second time varyingsignal, the ringer means providing the first time varying signal and thesecond time varying signal in response to the ring command. Theapparatus further includes detect means coupled with the interface meansfor detecting an AC impedance between the first conductor and the secondconductor in the presence of the ringing signal, the AC impedance havinga first value when the hook switch has its first state and a secondvalue when the hook switch has its second state, the detect meansproviding a ring trip indication when the AC impedance has one of thefirst value and the second value. The interface means, the ringer means,the detect means are preferably integrated in a common integratedcircuit.

The invention still further provides an apparatus for use with asubscriber line interface circuit in a telephone system, the subscriberline interface circuit being interposed between a telephone centraloffice and a subscriber line, the subscriber line being coupled with asubscriber telephone device and having a first conductor and a secondconductor. The apparatus comprises a first interface circuit coupledwith the subscriber line; a second interface circuit coupled with thetelephone central office and receiving a ring command from the telephonecentral office; a ringing generator coupled with the first interfacecircuit, the ringing generator providing a first ringing signal to thefirst conductor and a second ringing signal to the second conductor inresponse to the ring command, the first ringing signal and the secondringing signal being balanced, time varying signals. The apparatusfurther comprises an impedance detector coupled with the first interfacecircuit, the impedance detector detecting an AC impedance between thefirst conductor and the second conductor in the presence of the firstringing signal and the second ringing signal, the impedance detectorproviding a ring trip indication when the impedance is below apredetermined impedance threshold. The first interface circuit, thesecond interface circuit, the ringing generator and the impedancedetector are preferably integrated in a common integrated circuit, andthe subscriber telephone device produces a ringing indication inresponse to the first ringing signal and the second ringing signal.

The invention still further provides an improved subscriber lineinterface circuit for coupling to a subscriber line. The subscriber lineis coupled with a subscriber telephone device, the subscriber linehaving a first conductor and a second conductor. The subscriber lineinterface circuit includes a first interface circuit coupled with thesubscriber line; a second interface circuit coupled with the telephonecentral office and receiving a ring command from the telephone centraloffice; a ringing generator coupled with the first interface circuit,the ringing generator providing a first ringing signal to the firstconductor and a second ringing signal to the second conductor inresponse to the ring command, the first ringing signal and the secondringing signal being balanced, time varying signals; and an impedancedetector coupled with the first interface circuit, the impedancedetector detecting an AC impedance between the first conductor and thesecond conductor in the presence of the first ringing signal and thesecond ringing signal, the impedance detector providing a ring tripindication when the impedance is below a predetermined threshold. Thefirst interface circuit, the second interface circuit, the ringinggenerator and the impedance detector are integrated in a commonintegrated circuit. The subscriber telephone device produces a ringingindication in response to the first ringing signal and the secondringing signal.

It is therefore an advantage of the present invention to provide anapparatus and method for use in a subscriber line interface circuitwhich generates ringing signals for a subscriber telephone device anddetects an off hook state of the subscriber telephone device.

A further advantage of the present invention is to provide ringingsignals at a lower total voltage, relative to the prior art.

Yet a further advantage of the present invention is to provide a ringinggenerator which may be economically integrated in a silicon integratedcircuit.

Yet a further advantage of the present invention is to provide ringingsignals capable of ringing any telephone within the constraints ofindustry standards.

Further advantages and features of the present invention will beapparent from the following specification and claims when considered inconnection with the accompanying drawing illustrating the preferredembodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram illustrating a prior art centraloffice telephone system.

FIG. 2 is a schematic block diagram illustrating a fiber in the looptelephone system.

FIG. 3 is a schematic block diagram of a subscriber line interfacecircuit in which the apparatus of the present invention may be used.

FIG. 4a-4c are a diagram illustrating ringing signal voltage waveformsgenerated by the apparatus of FIG. 3.

FIG. 5a-5b are a diagram illustrating ringing signal current waveformsdetected by the apparatus of FIG. 3.

FIG. 6 is a flow diagram illustrating the preferred embodiment of themethod of the present invention.

For purposes of clarity and ease in understanding the present invention,like elements will be identified by like reference numerals in thevarious drawings.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic block diagram illustrating a prior art centraloffice telephone system 10. The central office telephone system 10includes a telephone central office 12 and at least one subscribertelephone device 14. The subscriber telephone device 14 is coupled tothe central office 12 by a subscriber line 16, the subscriber line 16including a tip conductor 18 and a ring conductor 20.

The telephone central office 12 controls switching of telephone callsplaced between the subscriber telephone device 14 coupled to thesubscriber line 16 and other subscriber telephone devices coupled toother subscriber lines 22 and 24. In particular, to signal the presenceof an incoming call to the subscriber telephone device 14, the centraloffice 12 generates ringing signals on the subscriber line 16.

To generate ringing signals, the central telephone office 12 includes aringing generator 26 coupled between the ring conductor 20 and theground potential 32. The tip conductor 18 is also switchably coupled toground potential 32 through a resistor 34. The telephone central office12 is adapted to couple the ringing generator 26 to the other subscriberlines 22, 24 to provide an indication of an incoming call to subscriberlines 22, 24.

The telephone central office 12 includes switches 27, 41 for selectivelycoupling the subscriber line 16 to either the ringing generator 26 or anormal telephone signal feed circuit 39. The normal feed circuit 39communicates telephone signals, rather than ringing signals, with thesubscriber 35 telephone device 14. To convey ringing signals to thesubscriber telephone device 14, the switch 27 decouples the normal feedcircuit 39 from the ring conductor 20 and couples the ringing generator26 to the subscriber line 16 and the switch 41 decouples the normal feedcircuit 39 from the tip conductor 18 and couples the tip conductor 18 toground potential 32 through resistor 34.

The telephone system 10 includes a subscriber line interface circuit 23coupled with the subscriber line 16 at terminals 15 and 17. Thesubscriber line interface circuit 23 may be located at the centraltelephone office 12, as illustrated in FIG. 1, or may be physicallyremote from the central telephone office in a private area branchexchange (PABX), as is known in the art. The subscriber line interfacecircuit 23 preferably couples the high-voltage analog subscriber line 16to lower voltage analog and digital circuits in the central telephoneoffice 12. Preferably the subscriber line interface circuit 23 supportsthe "BORSHT" functions (Battery feed, Over-voltage protection, Ringingsignal, Supervision of the subscriber line 16, Hybrid two-wire tofour-wire conversion, and Test). The telephone system 10 also includessubscriber line interface circuits 29, 31 coupled with the subscriberlines 22, 24, respectively.

The subscriber telephone device 14 includes a hook switch 36 forcoupling the tip conductor 18 with the ring conductor 20 at terminals 19and 21, respectively. The hook switch 36 generally has a first state anda second state for selectively coupling the tip conductor 18 and thering conductor 20. In its on hook state, the hook switch 36 couples thetip conductor 18 to a terminal 38. In the on hook state, the tipconductor is thus coupled to the ring conductor through a resistor 40,an inductor 42. In its off hook state, the hook switch 36 couples thetip conductor 18 to a terminal 46. Thus, in the off hook state, the tipconductor 18 is coupled by the hook switch 36 through a resistor or DCimpedance 48 to the ring conductor 20. When the subscriber telephonedevice 14 is not in use, the hook switch 36 has its first state andcouples the tip conductor 18 to the terminal 38. When the subscribertelephone device 14 is in use, the hook switch 36 has its second stateand couples the tip conductor 18 with the terminal 46. Specificsubscriber telephone devices may omit individual components such as theinductor 42 or the capacitor 44. In general, however, when anysubscriber telephone device is on hook, little or no DC current flows inthe subscriber line 16, and when any subscriber telephone device is offhook, DC current flows in the subscriber line 16.

In accordance with the prior art, the central telephone office 12couples the ringing generator 26 to the ring conductor 20 through theswitch 27 to supply ringing signals to the subscriber line 16. Theringing generator 26 includes an AC voltage source 28 and a DC offsetvoltage source 30. The ringing signals generated by the ringinggenerator 26 consist of an AC voltage generated by the AC voltage source28 superimposed on a DC voltage generated by the DC offset voltagesource 30. In accordance with the prior art and with industry standards,the AC voltage is typically a 20 Hz, 90 volt rms AC voltage and the DCvoltage is a 48 volt Dc voltage. The resulting waveform has amplitudepeaks of +77 volts and -173 volts, centered about the DC offset voltageof -48 volts.

Thus, the ringing generator 26 in accordance with the prior art must beable to handle voltage amplitude ranges greater than 250 volts. Becauseof this large voltage requirement, prior art ringing generators have notbeen economically implementable in silicon integrated circuit devices.Silicon devices which can withstand 250 volts, without experiencingJunction breakdown, are expensive to design and produce. Moreover,silicon manufacturing processes which may be able to handle 250 voltsbreakdown voltages are not readily combined with manufacturing processesfor producing silicon logic circuits, such as may be used forimplementing switching equipment used in the subscriber line interfacecircuit 23 by the central telephone office 12 or in a PABX.

The response of the telephone 14 to the prior art ringing signalsdepends on the state of the hook switch 36. When the hook switch 36 ison hook, coupling the tip conductor 18 to the terminal 38, the capacitor44 blocks the flow of DC current from the DC offset voltage source 30.However, the AC current generated by the AC voltage source 28 is passedby the capacitor 44, and AC current flows through the loop, whichincludes the capacitor 44, the inductor 42, the resistor 40, the hookswitch 36 and the resistor 34 in the central office 12. The subscribertelephone device 14 responds to this AC current by producing a ringingindication. For example, the inductor 42 might be the coil of a bell.The flow of the current in the loop causes the bell to ring in responseto the ringing signals. Alternatively, if the subscriber telephonedevice 14 is an electronic telephone and does not include a bell with aninductive coil, the electronic telephone detects the AC voltagegenerated by the AC voltage source 28 or its associated AC current andproduces a ringing indication in response thereto.

With the hook switch 36 in its second state, coupling the tip conductor18 to the terminal 46, AC current and DC current generated by the DCoffset voltage source 30 flows between the ring conductor 20 and the tipconductor 18 through the DC impedance 48. In accordance with the priorart, the central office 12 detected the flow of this DC current todetermine that the hook switch 36 was in its off hook state and that thesubscriber telephone device 14 was in use. This is known as ring tripdetection. In response to a ring trip detection, the central office 12interrupts the flow of the ringing signals to the subscriber telephonedevice 14 by opening the switch 27. Industry standards require that theringing signals be interrupted within a predetermined time periodfollowing ring trip detection, for example, 200 msec.

The prior art central office telephone system 10 thus performs ring tripdetection by examining the DC impedance of the subscriber line 16 at theterminals 15, 17 of the subscriber line interface circuit 23. This DCloop impedance also includes the impedance of the subscriber line 16itself. The DC impedance is tested by measuring either current flow orvoltage dropped by the subscriber line 16.

As shown in FIG. 1, the subscriber line 16 can be modelled as a loopresistance 50 in the tip conductor 18, and by a 10 K ohm resistor 52between the tip conductor 18 and the ring conductor 20, representingworst case leakage resistance between the tip conductor 18 and the ringconductor 20. In worst case conditions, where the subscriber line 16 isvery long, and the telephone 14 is located very distant from thesubscriber line interface circuit 23, the loop resistance 50 may be ashigh as 1500 ohms. In accordance with industry standards, when the hookswitch 36 is in its on hook state, the impedance seen across telephoneterminals 19, 21 is substantially equal to 1 REN (Ringer EquivalentNumber), where 1 REN is an impedance of 7000 ohms at 20 Hz. Further inaccordance with industry standards, the telephone system 10 mustfunction properly with up to five telephone devices such as subscribertelephone device 14 coupled to each subscriber line 16, 22, 24. Thus, inaccordance with industry standards, the system 10 must function normallywith up to 5 REN, or an impedance of 1400 ohms at 20 Hz. When the hookswitch 36 has its off hook state, the impedance seen between theterminals 19 and 21 may be substantially 100 to 430 ohms.

The value of the loop resistance 50 (R_(L)) is dependent on the lengthof the subscriber line 16, or the distance between the central office 12and the subscriber telephone device 14. Where the subscriber telephonedevice 14 is physically near the central office 12 (a short subscriberline), the loop resistance 50 has a value of substantially 0 ohms. Wherethe subscriber telephone device 14 is distant from the central office 12(a long subscriber line), the loop resistance 50 may be modelled with avalue of substantially 1500 ohms.

An example of a short subscriber line is the analog subscriber line usedin connection with a fiber in the loop (FITL) system. In such a systemone or more fiber optic cables couple the telephone central office withan optical network unit (ONU). The ONU is located within the immediatevicinity of the subscriber telephone devices serviced by the ONU. Thedistance from the ONU to the subscriber telephone device in such asystem is generally no more than a few city blocks.

In contrast, an example of a long subscriber line is the central officetelephone system 10 of FIG. 1, in which the subscriber line 16 may havea length on the order of miles. A central office telephone system mayhave a remote switching station coupling the subscriber telephonedevices to the central office. Even in this case, long subscriber linesmay be miles long.

The difference in impedances seen by the subscriber line interfacecircuit 23, between a high value when the subscriber telephone device 14has its on hook state and a low value when the subscriber telephonedevice 14 has its off hook state, is useful for ring trip detection.Ring trip detection includes determining when a subscriber telephonedevice is off hook, so that no ringing signals are conveyed to thesubscriber telephone device, and determining when a subscriber telephonedevice is taken off hook in response to ringing signals so that theringing signals may be interrupted and a call connected. The highimpedance of the on hook state and the low impedance of the off hookstate may be detected in a variety of ways. One known way is to supplycurrent to the subscriber line at a known ringing voltage. When thesubscriber telephone device is on hook, the current will have a firstvalue, and when the subscriber telephone device is off hook, the currentwill have a second, larger value. Ring trip detection occurs in responseto detecting this change in current. The change in current is anindication of the change in impedance.

Table 1 illustrates AC and DC impedances for both the on hook state andoff hook state of the hook switch 36, for both a short subscriber linesuch as subscriber line 16 and a long subscriber line such as subscriberline 16.

                  TABLE 1                                                         ______________________________________                                        SHORT LINES (5 REN)                                                           DC                     LONG LINES (5 REN)                                     IMPE-       AC         DC         AC                                          DANCE       IMPEDANCE  IMPEDANCE  IMPEDANCE                                   ______________________________________                                        OFF   100-400 Ω                                                                         100-400 Ω                                                                          1600-1900 Ω                                                                      1600-1900 Ω                         HOOK                                                                          ON    10 kΩ                                                                             1.4 k-7KΩ                                                                          10 kΩ                                                                            2.9 K-8.5 KΩ                        HOOK                                                                          ______________________________________                                    

Table 1 shows that, for both long subscriber lines and short subscriberlines, the DC loop impedance detected by the subscriber line interfacecircuit 23 can be readily used to distinguish the off hook conditionfrom the on hook condition of the hook switch 36. Even in the worst casesituation, with a long subscriber line, the off hook DC impedance is atmost substantially equal to 1900 ohms, and the on hook DC impedance is10 K ohms. The subscriber line interface circuit 23 can distinguish theon hook and off hook DC impedances by setting a detection threshold inthe middle of this broad range of DC loop impedance.

In contrast, Table 1 shows there is an overlap present in the AC loopimpedances. In the long lines situation, the AC off hook impedance maybe in the range of 1600 to 1900 ohms, which overlaps the possible rangefor the AC on hook impedance in the short lines situation, 1.4 K ohms to7 K ohms. Because of this overlap, the AC impedance may not be used toreliably distinguish the on hook state of the hook switch 36 from theoff hook state in a telephone system which involves both shortsubscriber lines and long subscriber lines. Thus, with the prior artsystems such as central office telephone system 10 illustrated in FIG.1, AC loop impedance cannot be used for ring trip detection. The ringinggenerator 26 must supply both AC and DC voltages, as discussed above,and the ringing generator 26, being required to accommodate a 250 voltrange of voltage, may not be economically integrated with the subscriberline interface circuit 23.

FIG. 2 shows a schematic block diagram illustrating a fiber in the loop(FITL) telephone system 60. In a FITL system, the central telephoneoffice 12 is coupled by a fiber optic cable 62 to an optical networkunit (ONU) 64. The central telephone office may also be coupled by otherfiber optic cables 66, 68 to other optical network units. The centraltelephone office 12 communicates with the ONU 64 digitally, using lightpulses conveyed over the fiber optic cable 62. The ONU 64 converts thelight pulses to electrical signals. The ONU includes a subscriber lineinterface circuit 23 for coupling the electrical signals with one ormore subscriber lines 72, 74. The subscriber lines 72, 74 may be coupledwith telephone equipment such as subscriber telephone device 14 (FIG.1). The subscriber lines 72, 74 convey analog electrical signals in amanner substantially identical to the subscriber line 16 illustrated inFIG. 1. For generating ringing signals and for effecting ring tripdetection, the subscriber line interface circuit 23 must conform to thesame industry standards imposed upon the prior art central officetelephone system 10 of FIG. 1.

The ONU 64 generally serves four to eight subscriber lines and isphysically located within a few city blocks of each of the associatedsubscriber line devices. By virtue of this close proximity, the worstcase impedance for the subscriber lines 72, 74, R_(L), is 100 ohms in anFITL system. Table 2 shows AC and DC impedances for both off hook and onhook conditions in an FITL system.

                  TABLE 2                                                         ______________________________________                                        SHORT LINES (5 REN)                                                           DC                     LONG LINES (5 REN)                                     IMPE-       AC         DC         AC                                          DANCE       IMPEDANCE  IMPEDANCE  IMPEDANCE                                   ______________________________________                                        OFF   100-400 Ω                                                                         100-400 Ω                                                                          200-500 Ω                                                                        200-500 Ω                           HOOK                                                                          ON    10 KΩ                                                                             1.4 K-7 KΩ                                                                         10 KΩ                                                                            1.5 K-7 KΩ                          HOOK                                                                          ______________________________________                                    

From Table 2, it is apparent that in an FITL system, there is no ACimpedance range overlap between the on hook, short line, situation andthe off hook, long line situation. Thus, for an FITL telephone system,ring trip detection can be made by detecting only the AC impedance ofthe subscriber line 72, 74 seen by the subscriber line interface circuit23. In accordance with the present invention, since AC signals are beingused to detect subscriber line impedance and to generate ringingsignals, no DC offset voltage need be generated by the ringinggenerator.

Elimination of the DC offset voltage generated by the ringing generatorprovides important advantages. The total voltage which must beaccommodated by devices associated with the ringing generator isreduced. This allows the ringing generator to be economicallyimplemented in a silicon integrated circuit. The silicon integratedcircuit is only required to accommodate voltages as great as the 90 Vrms ringing signal. Therefore, the silicon integrated circuit can bemanufactured at reduced cost compared with integrated circuits which canaccommodate voltages as large as the combination of the 90 V rms ringingvoltage and a DC offset voltage. Implementing the ringing generator in asingle silicon integrated circuit, preferably the same integratedcircuit containing the subscriber line interface circuit 23, reduces thenumber of devices and the number of interconnections required. This, inturn, reduces cost and improves reliability of the system.

Since in short loop applications the load impedance is significantlylarger than the line impedance, the attenuation of the ringing signalfrom the linecard to the subscriber equipment is small and the ringingvoltage at the subscriber line interface circuit or linecard can bereduced. In general, the line card including the subscriber lineinterface circuit 23 must provide a ringing voltage that is large enoughto provide a 40 V rms ringing signal at the telephone to ring thetelephone. If the loop is 1500 ohms, a 94 V rms signal with a 400 ohmsource impedance is required to provide 40 V rms to a 5 REN load at theend of the loop. However, if the loop impedance is 70 ohms and if thesource impedance can be reduced to, for example, 100 ohms, then a sourcevoltage of 45 V rms will provide 40 V rms to a 5 REN load.

FIG. 3 is a schematic block diagram of a subscriber line interfacecircuit 23 in which the apparatus of the present invention may be used.The subscriber line interface circuit 23 is coupled between a constantsignal source 80 and ground potential. The constant signal source 80preferably supplies a DC voltage to the subscriber line interfacecircuit 23. The constant signal source 80 is preferably a DC voltageknown as the battery voltage which is supplied to the subscriber lineinterface circuit 23 from external to the subscriber line interfacecircuit 23. The battery voltage preferably has a value in the range -21volts to -75 volts.

The subscriber line interface circuit 23 further includes a controlcircuit 86, a ringing generator 88, an impedance detector 90 and asubscriber line interface 92. The subscriber line interface 92 isadapted to be coupled to the subscriber line 72, including a tipconductor 94 and a ring conductor 96. The subscriber line interface 92may include current limiting resistors and other interface components(not illustrated in FIG. 3). The subscriber line interface 92 thus formsan interface means for coupling the subscriber line interface circuit 23with the subscriber line 72.

The ringing generator 88 includes a first amplifier 98 and a secondamplifier 100. The first amplifier 98 and the second amplifier 100 areeach coupled with the constant signal source 80 and to ground potential.The first amplifier 98 has an input 108 coupled with an output 110 ofthe control circuit 86. The first amplifier 98 has an output 112 coupledwith the tip conductor 94. The second amplifier 100 has an input 120coupled with an output 122 of the control circuit 86. The secondamplifier 100 has an output 124 coupled with the ring conductor 96. Theringing generator 88 thus provides a ringer means for providing a timevarying ringing signal to the subscriber line in response to a receivedcontrol signal.

The control circuit 86 has a ring input 130, a control input 87, a firstfeedback input 132 and a second feedback input 134. The control circuit86 includes digital logic circuitry for controlling the operation of thesubscriber line interface circuit 23. The control circuit responds tocommands received at control input 87. The first feedback input 132 iscoupled to the tip conductor 94. The second feedback input 134 iscoupled to the ring conductor 96. Preferably, the ring input 130 and thecontrol input 87 are adapted to receive control signals having standardsignal levels, such as CMOS signal levels.

In operation, the subscriber line interface circuit 23 receives commandand control information conveyed from the central office 12 to inputsincluding the ring input 130 and the control input 87. The controlcircuit 86 receives the command and control information which mayinclude a ring command directing the subscriber line interface circuit23 to provide ringing signals to the subscriber line 72 and to detectfor ring trip. In response to a ring command received at the ring input130, the control circuit 86 provides control signals at outputs 110 and122 which cause the first amplifier 98 and the second amplifier 100 togenerate a first time varying signal and a second time varying signal,respectively. The first time varying signal and the second time varyingsignal are conveyed to the tip conductor 94 and the ring conductor 96,respectively.

In accordance with the present invention, the first time varying signalconveyed by the tip conductor 94 and the second time varying signalconveyed by the ring conductor 96 are balanced, time varying signals.That is, the first time varying signal and the second time varyingsignal have substantially equal amplitudes of substantially oppositepolarity. FIG. 4 is a diagram illustrating ringing signal voltagewaveforms generated by the apparatus of FIG. 3. FIG. 4(a) is a diagramof a waveform representing the first time varying ringing signalgenerated at the tip conductor 94 by the first amplifier 98. FIG. 4(b)is a diagram of a second time varying ringing signal generated at thering conductor 96 by the second amplifier 100. In FIG. 4(a), during afirst phase defined by the time intervals t₃ -t₂, t₈ -t₇, etc., thewaveform has an amplitude substantially equal to the absolute value ofthe battery voltage V_(BAT) provided by the constant signal source 80.In FIG. 4(b), during a second phase defined by the time intervals t₁-t₀, t₅ -t₄, etc., the waveform provided to the ring conductor 96 has anamplitude substantially equal to the absolute value of the batteryvoltage V_(BAT) provided by the constant signal source 80. Over apredetermined time period, the amplitude of the first waveform and theamplitude of the second waveform, measured at each instant in time, sumto a predetermined constant level. The first waveform and the secondwaveform thus represent balanced signals. Preferably, non-zero timeintervals defined by the time intervals t₂ -t₁, t₄ -t₃, t₅ -t₆, t₈ -t₇,etc., are provided during which the polarities of the first and secondtime varying signals are switched. The first time varying signal and thesecond time varying signal preferably have a frequency of 20 Hz toconform with industry standards for ringing signals.

Referring again to FIG. 3, while the first time varying signal suppliedto the tip conductor 94 and the second time varying signal supplied tothe ring conductor 96 may be generated by any apparatus known to thosehaving ordinary skill in the art, the first amplifier 98 and the secondamplifier 100 represent an exemplary apparatus for generating ringingsignals in accordance with the present invention. In response to acontrol signal received at the input 108, the first amplifier 98alternately couples the tip conductor 94 to V_(BAT), the DC voltagesupplied by the constant signal source 80, and to ground potential.Similarly, in response to a control signal received at the input 120,the second amplifier 100 alternately couples the ring conductor 96 toV_(BAT) and to ground potential. By selectively reversing polaritiesbetween V_(BAT) and ground, the waveform of FIG. 4(a) and the waveformof FIG. 4(b) may be generated.

The potential difference between these two voltages (the waveforms ofFIG. 4(a) and FIG. 4(b)) is represented by the waveform of FIG. 4(c).The waveform of FIG. 4(c) is supplied to the tip and ring conductors 94,96 as a time varying ringing signal. The waveform of FIG. 4(c) swingsbetween a voltage substantially equal to -V_(BAT) and the absolute valueof the battery voltage, +V_(BAT). The upper and lower limits of thewaveform may be limited to less than the battery voltage or its absolutevalue by, for example, 2.5 volts, which is the saturation limit of thefirst amplifier 98 and the second amplifier 100. The constant signalsource 80 preferably supplies V_(BAT) as a DC voltage in the range -21volts to -75 volts. Thus, the potential difference between the firsttime varying signal, conveyed to the tip conductor 94, and the secondtime varying signal, conveyed to the ring conductor 96, is a timevarying signal of at least 90 volts rms. This satisfies industrystandards for ringing signals.

In accordance with industry standards, ringing signals do not have to beperfect sine waves. For short loop applications, such as fiber in theloop and private area branch exchanges, industry standards require thatthe crest factor (the peak voltage to rms voltage ratio) of the ringingsignal must be greater than 1.2 and less than 1.6. For long loopapplications, industry standards require a crest factor greater than1.35 and less than 1.45. A perfect sine wave has a crest factor of 1.41.Industry standards specifying a crest factor near 1.41 (a pure sinewave) are related to the problem of harmonic signal generation andcrosstalk generated in adjacent lines. However, this is a moresignificant problem for a central office telephone system, asillustrated in FIG. 1, than for FITL applications, since central officeapplications have many subscriber lines running together in a cablebundling over a long distance. FITL applications, on the other hand, runshort distances and are distributed so that fewer cables are bundledtogether. Moreover, the apparatus of the present invention uses muchlower ringing signal voltages than the prior art central office systemillustrated in FIG. 1, so less coupling of noise will occur betweensubscriber lines. Still further, an apparatus in accordance with thepresent invention uses differential or balanced ringing as opposed tosingle-ended or unbalanced ringing.

In unbalanced ringing, either the tip conductor or the ring conductor iscoupled to ground and the entire ringing signal, including any DCoffset, is provided to the other conductor. In balanced ringing, aportion of the ringing signal, preferably one-half the total amplitudeof the ringing signal, is applied to each conductor, tip and ring.

Balanced ringing generates much less crosstalk than unbalanced ringingsince, with balanced ringing, both conductors each have one-half the ACsignal and they tend to cancel the coupling effects of each other on afirst order level. Therefore, a crest factor close to 1.2 is acceptablefor FITL applications, and a ringing generator for FITL applicationsneed not generate a pure sine wave. A trapezoid wave, as illustrated inFIG. 4(c), has a crest factor less than 1.41 and close to 1.2.

Use of a square wave or trapezoid wave, which has more energy than asine wave of the same frequency, allows use of lower peak amplituderinging signals. This means the ringing signals may be generated by thesame silicon integrated circuit used to provide logic and controlfunctions. Thus, the ringing generator 88 and the control circuit 86 maybe combined on a single integrated circuit powered by a single batteryvoltage.

To increase the crest factor of the signals supplied to the tipconductor 94 and the ring conductor 96, the ringing generator 88 mayinclude filter circuits between the output 112 of the first amplifier 98and the tip conductor 94 and between the output 124 and the ringconductor 96 (not shown in FIG. 3). Such filter circuits would filterout unwanted harmonic frequencies to produce ringing signals moreclosely approximating a pure sine wave. However, as indicated above, thetrapezoid wave produced by the amplifier 98 and the amplifier 100 shouldbe adequate for most applications, including FITL applications, whileavoiding additional costs of the components required to include one ormore filter circuits in the ringing generator 88. Preferably, thecharacteristics of the trapezoid wave produced by the amplifiers 98,100, such as rise and fall times and peak to peak amplitudes, arecontrollable from external to the subscriber line interface circuit 23.

Referring again to FIG. 3, the impedance detector 90 includes a firstcurrent detector 150, a second current detector 152, a filter 154, aring trip detector 156 and a DC offset circuit 158. The impedancedetector 90 detects the AC impedance between the tip conductor 94 andthe ring conductor 96 in the presence of the ringing signal generated bythe ringing generator 88. The impedance detector 90 thus forms a detectmeans for detecting an impedance of the subscriber line in the presenceof a time varying ringing signal and providing a ring trip indicationwhen the impedance is below a predetermined impedance threshold.

While any apparatus known in the art for detecting an AC impedance maybe employed, the impedance detector 90 illustrated in FIG. 3 is anexemplary impedance detector in accordance with the present invention.The first current detector 150 includes an input 160 for detecting thecurrent supplied by the amplifier 98 to the tip conductor 94. The firstcurrent detector 150 rectifies the current detected at the input 160 andprovides a signal representative of the rectified current to an output164. The second current detector 152 includes an input 162 for detectingthe current supplied by the amplifier 100 to the ring conductor 96. Thesecond current detector 152 rectifies the current detected at the input162 and provides a signal representative of the rectified current at theoutput 166. The signals at the outputs 164, 166 are combined to producea rectified net current which is provided to the filter 154. The filter154 includes a resistor 170 and a capacitor 172 for filtering unwantedfrequency components of the combined signal. The DC offset circuit 158includes a resistor 174 coupled to the constant signal source 80 forreceiving the battery voltage, V_(BAT). The DC offset circuit 158 adds aDC offset to the combined signal representative of the rectified currentand the result is provided to an input 168 of the ring trip detector156.

The ring trip detector 156 preferably includes a comparator 183 forcomparing the value received at the input 168 with a predeterminedthreshold supplied at an input 176. The ring trip detector 156 has anoutput 178 coupled to an input 180 of the control circuit 86. When thesignal received at the input 168 of the ring trip detector 156 exceedsthe predetermined threshold, indicating the currents supplied to the tipconductor 94 and the ring conductor 96 have increased due to thereduction in AC impedance as the subscriber telephone device coupledwith the subscriber line 72 is taken off hook, the ring trip detector156 provides a ring trip indication at the output 178 to the input 180of the control circuit 86. In response to the ring trip indication, thecontrol circuit 86 interrupts the control signals supplied at theoutputs 110 and 122. In response to the interrupt of the control signal,the first amplifier 98 and the second amplifier 100 interrupt theringing signals supplied to the tip conductor 94 and the ring conductor96.

Preferably, the ringing generator 88, the impedance detector 90 and thesubscriber line interface 92 are integrated on a common integratedcircuit 181. In addition, the control circuit 86 may also be integratedon the common integrated circuit 86.

FIG. 5 is a diagram of ringing signal current waveforms detected by theapparatus of FIG. 3. FIG. 5(a) illustrates the rectified currentwaveform detected by the impedance detector 90 when ringing signals aresupplied by the ringing generator 88 to a subscriber line coupled to asubscriber telephone device in the on hook state. As illustrated in FIG.5(a), the detected, rectified current waveform is generally a sinusoidhaving peak value of approximately 45 mA. FIG. 5(b) represents therectified current waveform detected by the impedance detector 90 whenringing signals are supplied to a subscriber line coupled to asubscriber telephone device in the off hook state. As can be seen inFIG. 5(b), the rectified current waveform has the form of a trapezoidwave having a peak value of approximately 110 mA. The difference betweenthe peak values of the waveforms illustrated in FIGS. 5(a) and 5(b) issufficient to allow accurate detection of a change in the AC impedanceof a subscriber line coupled to the ringing generator and thus allowaccurate on hook detection.

FIG. 6 is a flow diagram illustrating the preferred embodiment of themethod of the present invention. The method begins at step 200. At step202, ringing signals are provided to a subscriber telephone line whichis coupled with a subscriber telephone device. The ringing signalspreferably include a first time varying signal and a second time varyingsignal.

The method continues at step 204, where the AC impedance of thesubscriber line is detected. This may be accomplished by any suitabletechnique known to those of ordinary skill in the art; preferably, thepeak current supplied to the subscriber line is detected as describedabove in connection with FIGS. 3 and 5. When the peak current issupplied at a known time varying voltage, the AC impedance can bedetermined from the peak current.

At step 206, the AC impedance of the subscriber line in the presence ofthe ringing signals is compared with a predetermined threshold. If theAC impedance is above the predetermined threshold, it is determined thatthe subscriber telephone device is on hook. If the detected AC impedanceis above the predetermined threshold, the method returns to step 202 andringing signals continue to be generated. If the detected AC impedanceis below the predetermined threshold, it is determined that thesubscriber telephone device is on hook and the method continues at step208, where the ringing signals are interrupted. The method terminates atstep 210.

The present invention allows reduction of the DC operating voltagerequired for a subscriber line interface circuit that is able to ringtelephone devices without the use of an external ringing generator and aring delay. Preferably the telephone lines with which the invention isused are relatively short telephone lines. Short telephone lines haverelatively low loop resistance. In FITL (fiber in the loop)applications, loop resistance is less than 100 ohms. This is also truein hybrid fiber coax and other set-up or box-on-the-side-of-the-homeapplications. In some areas, this loop resistance may increase to a fewhundred ohms. Also key telephone systems and some small PABXes (privatearea branch exchanges) have short loops. One key characteristic of ashort loop is that the impedance of the loop plus the off hook impedanceof the telephone device (which is less than 430 ohms) is significantlyless than the on-hook impedance of the ringing circuit in the telephoneset. In the United States, FITL systems specify the maximum ringing loadas 5 REN (ringer equivalent number) or about 1400 ohms plus a 40 uFcapacitor. However, other countries have lighter loads. For example, theUnited Kingdom specifies a maximum ringing load of 3 REN. Many PABXesonly have a 1 or 2 REN requirement. When this short loop criterion ismet there are several benefits.

One benefit is that loop detection can be accomplished by looking at theAC impedance or current rather than the DC impedance/current. Therefore,it is possible to reduce or eliminate the DC component of the ringingsignal.

A second benefit is that short loop applications can use balancedringing because they do not have to support multi-party ringingapplications. Balanced ringing has two major benefits. First of all, itreduces the DC maximum voltage that is required. An unbalanced ringingsystem requires that the DC operating voltage be larger than thepeak-to-peak value of the AC signal or the peak value of the AC signal,and the DC offset, whichever is greater, while a balanced systemrequires a DC operating voltage that is greater than the peak AC signalplus the DC offset. If the DC offset is reduced to zero, a balancedsystem requires about one half of the operating voltage of the peak ACsignal of the source voltage. Short loop systems have a much lowersource voltage than long loop systems, so the combination of thesefactors greatly reduces the DC operating voltage requirements. Balancedringing injects much less crosstalk into adjacent lines due to itsbalanced nature. In addition, in short loop applications, short loopsand lower ringing voltages produce much less coupling than long loopswith high ringing voltages. Finally, short loops do not have many otherlines running along side the loop so coupling is less of a problem.These factors allow reduction of the crest factor of the ringing signaland use of a trapezoidal waveform as opposed to a sine wave. A 1.25crest factor (to provide adequate margin if the spec is 1.2 minimum)reduces the peak signal by about 10% relative to a sine wave and reducesthe DC operating voltage by another 10%. A trapezoidal waveformgenerator is much easier and less expensive to implement than a sinewave generator.

The combination of these techniques allows reduction of the operatingvoltage requirements from almost 300 V for a central office-typeapplication down to about 60 V for a short loop application. Use of asomewhat higher voltage allows a larger ringing signal or a longer loopto be driven. This voltage range can be supported by low cost, highreliability integrated circuit technologies.

It is to be understood that, while the detailed drawing and specificexamples given describe preferred embodiments of the invention, they arefor the purpose of illustration, that the apparatus of the invention isnot limited to the precise details and conditions disclosed and thatvarious changes may be made therein without departing from the spirit ofthe invention which is defined by the following claims.

I claim:
 1. Apparatus for use with a subscriber line interface circuitin a telephone system, the subscriber line interface circuit beinginterposed between a telephone central office and a subscriber line, thesubscriber line being coupled with a subscriber telephone device, theapparatus comprising:an interface for coupling the apparatus with saidsubscriber line; a ringer coupled with said interface for providing atime varying ringing signal to said subscriber line in response to areceived control signal; and a detector coupled with said interface fordetecting only an AC impedance of said subscriber line in the presenceof said time varying ringing signal and providing a ring trip indicationwhen said AC impedance is below a predetermined impedance threshold. 2.Apparatus for use with a subscriber line interface circuit as recited inclaim 1 further comprising a controller coupled with said ringer andwith said detector for providing said control signal to said ringer andreceiving said ring trip indication from said detector, said controllerinterrupting said control signal in response to receiving said ring tripindication.
 3. Apparatus for use with a subscriber line interfacecircuit as recited in claim 12 wherein said controller interrupts saidcontrol signal within a predetermined time of receiving said ring tripindication.
 4. Apparatus for use with a subscriber line interfacecircuit as recited in claim 3 wherein said controller interrupts saidcontrol signal within 200 milliseconds of receiving said detectindication.
 5. Apparatus for use with a subscriber line interfacecircuit as recited in claim 1 wherein said subscriber line has a DCimpedance and an AC impedance and wherein said detector detects said ACimpedance.
 6. Apparatus for use with a subscriber line interface circuitas recited in claim 1 wherein said time varying ringing signal has acharacteristic,. and wherein said detector comprises a comparatorcoupled with said interface for receiving said time varying ringingsignal, sensing said characteristic and producing said ring tripindication when said characteristic exceeds a predetermined threshold.7. Apparatus for use with a subscriber line interface circuit as recitedin claim 6 wherein said time varying ringing signal comprises analternating current provided at a predetermined voltage, and whereinsaid characteristic comprises a first amplitude of said alternatingcurrent, and wherein said comparator means provides said ring tripindication when said first amplitude exceeds a predetermined currentthreshold.
 8. Apparatus for use with a subscriber line interface circuitas recited in claim 1 wherein said time varying ringing signal comprisesan alternating current provided at a predetermined voltage, saidalternating current having a first amplitude, and wherein said detectorcomprises a current detector coupled with said interface for detectingsaid first amplitude and a comparator coupled with said current detectorfor providing said ring trip indication when said first amplitudeexceeds a predetermined current threshold.
 9. Apparatus for use with asubscriber line interface circuit as recited in claim 8 wherein saidsubscriber line comprises a first conductor and a second conductor, saidfirst conductor and said second conductor being coupled through saidsubscriber telephone device, said first conductor receiving saidalternating current from said ringer , said second conductor conductinga return current including a portion of said alternating current, saidreturn current having a second amplitude, and wherein said currentdetector detects said alternating current and said return current andgenerates an output, said output having a predetermined relationship tosaid first amplitude and said second amplitude, said comparatorproviding said ring trip indication when said output exceeds apredetermined current threshold.
 10. Apparatus for use with a subscriberline interface circuit as recited in claim 1 wherein said subscriberline comprises a first conductor and a second conductor, said firstconductor and said second conductor being coupled through saidsubscriber telephone device, and wherein said time varying ringingsignal comprises a first time varying ringing signal comprises a firsttime varying ringing signal component and a second time varying ringingsignal component, said ringer providing said first time varying ringingsignal component to said first conductor and said second time varyingsignal component to said second conductor.
 11. Apparatus for use with asubscriber line interface circuit as recited in claim 10 wherein saidfirst time varying ringing signal component and said second time varyingringing signal component are balanced.
 12. Apparatus for use with asubscriber line interface circuit as recited in claim 11 wherein saidfirst time varying ringing signal component has a first amplitude andsaid second time varying signal component has a second amplitude, andwherein, over a predetermined time period, said first amplitude and saidsecond amplitude sum to a predetermined constant level.
 13. Apparatusfor use with a subscriber line interface circuit as recited in claim 12wherein said predetermined constant level is substantially zero. 14.Apparatus for use with a subscriber line interface circuit as recited inclaim 11 wherein said first time varying ringing signal component has afirst amplitude and said second time varying ringing signal componenthas a second amplitude, said first amplitude being substantially equalto said second amplitude, and wherein said first time varying signalcomponent has a first frequency and said second time varying signalcomponent has a second frequency, said first frequency beingsubstantially equal to said second frequency, and wherein said secondtime varying ringing signal component lags said first time varyingringing signal component by a phase angle substantially equal to 180degrees.
 15. Apparatus for use with a subscriber line interface circuitas recited in claim 1 wherein said subscriber telephone device includesa hook switch having an on hook position and an off hook position, saidAC impedance having a first value when said hook switch is in said onhook position and a second value when said hook switch is in said offhook position.
 16. Apparatus for use with a subscriber line interfacecircuit as recited in claim 1 wherein said interface means, said ringerand said detector are integrated in a common integrated circuit. 17.Apparatus for coupling a subscriber line with a digital signal path in atelephone system, the apparatus receiving information including a ringcommand from said digital signal path, said subscriber line beingcoupled with a subscriber telephone device, said subscriber telephonedevice including a signal generator and hook switch, said subscriberline having a first conductor and a second conductor, said hook switchhaving a first state and a second state, said signal generatorgenerating a ringing indication in response to a ringing signal on saidsubscriber line when said hook switch is in said second state, theapparatus comprising:an interface for coupling the apparatus with saidsubscriber line; a ringer coupled with said interface for providing saidringing signal to said subscriber line, said ringing signal includingonly a first time varying signal and a second time varying signal, saidringer providing said first time varying signal and said second timevarying signal in response to said ring command; and a detector coupledwith said interface for detecting only an AC impedance between saidfirst conductor and said second conductor in the presence of saidringing signal, said AC impedance having a first value when said hookswitch has said first state and a second value when said hook switch hassaid second state, said detector providing a ring trip indication whensaid AC impedance has one of said first value and said second value,said interface, said ringer and said detector being integrated in acommon integrated circuit.
 18. Apparatus for coupling a subscriber linewith a digital signal path as recited in claim 17 wherein the apparatusfurther comprises a controller coupled with said ringer and saiddetector for receiving said ring trip indication and for interruptingsaid first time varying signal and said second time varying signal inresponse to said ring trip indication.
 19. Apparatus for coupling asubscriber line with a digital signal path as recited in claim 18wherein said interface, said ringer and said detector are integrated ina common integrated circuit.
 20. Apparatus for coupling a subscriberline with a digital signal path in a telephone system as recited inclaim 17 wherein said ringer provides said first time varying signal tosaid first conductor and said second time varying signal to said secondconductor and wherein said first time varying signal and said secondtime varying signal are balanced.
 21. Apparatus for coupling asubscriber line with a digital signal path in a telephone system asrecited in claim 20 wherein the apparatus further comprises means forcoupling the apparatus to a constant signal source, said constant signalsource supplying a DC voltage, and wherein said ringer includes a timevarying signal generator for receiving said DC voltage and selectivelyproviding said DC voltage to said first conductor and said secondconductor to generate said first time varying signal and said secondtime varying signal.
 22. Apparatus for use with a subscriber lineinterface circuit in a telephone system, the subscriber line interfacecircuit being interposed between a telephone central office and asubscriber line, the subscriber line being coupled with a subscribertelephone device, the subscriber line having a first conductor and asecond conductor, the apparatus comprising:a first interface circuit forcoupling with said subscriber line; a second interface circuit forcoupling with said telephone central office and receiving a ring commandfrom said telephone central office; a ringing generator coupled withsaid first interface circuit, said ringing generator providing a firstringing signal to said first conductor and a second ringing signal tosaid second conductor in response to said ring command, said firstringing signal and said second ringing signal being balanced, timevarying signals, said subscriber telephone device producing a ringingindication in response to said first ringing signal and said secondringing signal; and an AC impedance detector coupled with said firstinterface circuit, said AC impedance detector detecting only an ACimpedance between said first conductor and said second conductor in thepresence of said first ringing signal and said second ringing signal,said AC impedance detector providing a ring trip indication when saidimpedance is below a predetermined threshold; said first interfacecircuit, said second interface circuit, said ringing generator, and saidAC impedance detector being integrated in a common integrated circuit.23. Apparatus for use with a subscriber line interface circuit in atelephone system as recited in claim 22 wherein the apparatus furthercomprises a controller coupled with said ringing generator, with said ACimpedance detector, and with said second interface circuit, saidcontroller conveying said ring command from said second interfacecircuit to said ringing generator and receiving said ring tripindication from said AC impedance detector, said controller interruptingsaid ring command in response to receiving said ring trip indication.24. Apparatus for use with a subscriber line interface circuit in atelephone system as recited in claim 22 wherein the apparatus furthercomprises a third interface circuit configured to be coupled with aconstant voltage source and said ringing generator and for conveying aDC voltage from said constant voltage source to said ringing generator,and wherein said ringing generator selectively provides said DC voltageto said first conductor and said second conductor to provide said firstringing signal and said second ringing signal.
 25. Apparatus for usewith a subscriber line interface circuit in a telephone system asrecited in claim 22 wherein said first ringing signal comprises a firstalternating current having a first amplitude and said second ringingsignal comprises a second alternating current having a second amplitude,said first alternating current and second alternating current beingprovided as a predetermined voltage, and wherein said AC impedancedetector includes a current detector which senses said first amplitudeand said second amplitude, said AC impedance detector providing saidring trip indication when said first amplitude and said second amplitudeexceed a predetermined current threshold.
 26. An improved subscriberline interface circuit for coupling to a subscriber line, the subscriberline being coupled with a subscriber telephone device, the subscriberline having a first conductor and a second conductor, the subscriberline interface circuit comprising:a first interface circuit for couplingwith said subscriber line; a second interface circuit for coupling withsaid telephone central office and receiving a ring command from saidtelephone central office; a ringing generator coupled with said firstinterface circuit, said ringing generator providing a first ringingsignal to said first conductor and a second ringing signal to saidsecond conductor in response to said ring command, said first signal andsaid second ringing signal being balanced, time varying signals, saidsubscriber telephone device producing a ringing indication in responseto said first ringing signal and said second ringing signal; and an ACimpedance detector coupled with said first interface circuit, said ACimpedance detector detecting only an AC impedance between said firstconductor and said second conductor in the presence of said firstringing signal and said second ringing signal, said AC impedancedetector providing a ring trip indication when said impedance is below apredetermined impedance threshold; said first interface circuit, saidsecond interface circuit, said ringing generator, and said AC impedancedetector being integrated in a common integrated circuit.
 27. Animproved subscriber line interface circuit for coupling to a subscriberline as recited in claim 26 wherein the subscriber line interfacecircuit further comprises a controller coupled with said ringinggenerator, with said AC impedance detector, and with said secondinterface circuit, said controller conveying said ring command from saidsecond interface circuit to said ringing generator and receiving saidring trip indication from said AC impedance detector, said controllerinterrupting said ring command in response to receiving said ring tripindication.
 28. An improved subscriber line interface circuit forcoupling to a subscriber line as recited in claim 27 wherein said firstinterface circuit, said second interface circuit, said ringinggenerator, said AC impedance detector and said controller are integratedin a common integrated circuit.